1. Field of Invention
This invention relates to backpacks, specifically to the hipbelt.
2. Description of Prior Art
Heretofore, backpack hipbelts have been set forth in U.S. Pat. No. 4.015,759 to Dreissigacker et al. (1977), U.S. Pat. No. 4,449,654 to Cappis (1982), U.S. Pat. No. 5,184,764 to Orovan et al. (1989), U.S. Pat. No. 5,547,461 to Levis (1994) and U.S. Pat. No. 5,429,287.
Dreissigacker et al. labeled the X,Y and Z axes as the axis perpendicular to the line of travel, vertical and the line of travel respectively. Herein the X axis is parallel to a line of horizontal travel the Y axis is perpendicular to the other axes and the Z axis is vertical. I have labeled the axes according to the scientific and engineering convention of labeling the vertical axis Z and the other reference direction with the primary axis, X. For purposes of discussion, hipbelts will be described as consisting of two parts. All hipbelts I have seen have at least two points connecting them to the backpack. The two points farthest apart in the Y axis will be referred to as the hipbelt pivots. They are not always designed to be pivots, but the fact that the belt is flexible and free outside these points means there is some pivoting. The hipbelt pivots will be used to divide the hipbelt into two portions. The portion rearward and/or between the two points will be referred to as the rear panel. The portion extending laterally and/or bending around forward of the pivots will be referred to as the front panel. In many instances the rear panel and the front panel will be continuous through the pivot points. This is most often true for side attachment hipbelts. These hipbelts usually are a continuous fabric covered foam pad that extends around the entire circumference of the user. The pivot points are usually a grommet attached in some way to that hipbelt at the side of the user.
Over the past 30 years hipbelts have developed into an essential part of a backpack. Their capability to transfer weight to the hips greatly increases the endurance of the hiker. To make the hipbelt more comfortable the manufacturers have made them wide and soft and yet they can still cause discomfort. I believe that to reduce this discomfort they design their packs to place some of the weight on the shoulders. This is reflected in their elaborately designed shoulder straps. There seems to be a need for a hipbelt that can comfortably transfer more weight to the hips.
The prior art shows that the idea of a semi-rigid rear panel is not new. In fact, I believe that 20 years ago, the taut belt type rear panel dominated the market. However, the art has tended away from the taut belt or any type of semi-rigid rear panel. The art has steadily tended towards soft, flexible hipbelts that conform to the waist. The idea seems to be to cushion all points of the waist from the stress of the load. This might work well except for the fact that in so doing the hipbelt is tightened over fat and muscle found lateral of the sacrum and lumbar vertabrae. Fat is not a structurally sound material. Muscle has some ability to resist deformation under stress but it also expands and contracts with each step. The hipbelt slides down while the muscle is contracted and then cinches on the muscle when it attempts to expand. This cinching reduces the muscles ability to expand and thereby its ability to do work. It may also squeeze nerves within and undemeath the muscle. This may seriously affect the muscles and cause fatigue.
The taut belt rear panel still found on some external frame packs can effectively bridge across the gluteous muscles. The rear panel is attached to the frame for its full width so it cannot pivot relative to the backpack at its pivot points. The angle with which the panel meets the back is determined by the angle designed into the pack frame plus the angle imparted to it due to bending of the back. Therefore the panel will only intermittently parallel the back where it contacts it. When it is not parallel it will be applying pressure primarily along its upper or lower edges. This amounts to reducing the area of contact and thereby increasing pressure. I believe increasing pressure can only increase the likelyhood of damage to the tissues and discomfort to the user. Some packs have a rear panel that is loose rather than taut. The slack allows some rotation of the panel even though that requires torsional flexure of a band of fabric about 3 inches wide. Kelty, Inc. of St. Louis, Mo., produces the "Yukon", a backpack is of this type. The only gain in rotation it offers is at the expense of bridging ability. In my opinion, it is not possible to achieve a significant amount of rotation by allowing slack without loosing the bridging capability. I think this is because the back in the gluteal region is approximately flat in the central half and only slightly curved lateral of the central half. Obesity may preferentially increase the amount of fat over the lateral areas and might even require a rear panel that is concave towards the back to bridge across the lateral areas. That is a loose belt simply has too much of a concave curve to bridge adequately. Many modem packs have stabilizer straps. These are adjustable straps that connect to the side of the pack and to the hipbelt at a point a few inches forward of the pack. If tight, they may in affect bridge some af the gluteal region. In essence, the stabilzer straps have formed new and wider pivot points. The instruction page for the Yukon backpack says "The stabilizer straps on the side of the belt must not be over tightened. Leave them slightly loose".
Dreisigacker et al. show a taut belt design that cannot pivot relative to the backpack around the Y axis. Furthermore, if it did pivot around the Y axis at the same joint that allows pivoting around the other two axes it would be uncomfortable, in my experience. The weight of the load would tend to jackknife the frame around that joint. At the very least, this would cause the taut belt to dig in along its lower edge. To avoid this problem it is neccessary to locate the Y pivot at or forward of the users back/cushion interface. One example of the Y pivot being located in front of that interface is side attachment packs.
Side attachment pack frames attach to the hipbelt lateral of the center of the pelvic region. In this configuration both the front panel and the rear panel pivot with respect to the backpack. Such hipbelts usually are flexible enough that the rear panel pivots through flexure relative to the front panel also. This allows the rear panel to match the angle of the back at all times. Another advantage of the hipbelt pivots being located this far forward is that the rear panel can contact the back higher on the back; higher on the sacrum and ilium. This can feel more comfortable and avoids some nerves on the posterior of the sacrum which may be compressed by some hipbelts. However, all side attachment hipbelts to date have a flexible, comfortable rear panel. The fact that the weight is applied on the sides may mean that the force on the rear panel has a larger component in the forward direction. This may cinch around the gluteus and pyriformis muscles with more force. In turn, this may lead to discomfort. In my experience, hipbelts worn too low make the muscles feel tired too quickly. I think that some muscles or portions of muscles are made weaker either by cinching and/or nerve compression. Then, the remaining muscle mass quickly becomes tired due to compensating for the ineffective muscle. It is difficult to distinguish between ordinary muscle tiredness and the tiredness due to a hipbelt worn too low. I believe that is why side attachment hipbelts have never been improved or gained in popularity. A further difficulty of side attachment hipbelts is that they usually require a system to adjust the location of the Y-pivots. One complex example is found in the hip-brace in U.S. Pat. No. 5,429,287. This incurs more expense and weight and can be an awkward system.
Modern internal frame packs often have a sheet of semi-rigid plastic for a frame. They have a rear panel that is sewn to the pack sack. The configuration is such that in some cases either the plastic sheet bridges or it holds the belt somewhat taut and some bridging is achieved in that way. In the former case, the sheet in the gluteal region may flex a little around the Y axis but essentially takes an angle that parallels the back. This is seen from the premise that the sheet was rigid enough to bridge across the gluteal region. In the latter case, the configuration is similar to an older style taut belt pack and prone to drawbacks inherent in that design.